Re: How to get rid of VC warning C4239?

Bart van Ingen Schenau <>
Sat, 7 Jul 2007 15:47:33 CST
ZHENG Zhong wrote:

For the first choice... One of the goal of the basic_xml_node class
template is to make the interface pointer-free. For example, i have a
MsxmlNode class that uses msxml, and a LibxmlNode that uses libxml.
Both classes implement a member function called parent, like this:

class LibxmlNode {
    LibxmlNode* parent();

class MsxmlNode {
    MsxmlNode* parent();

Note that this use of pointers does not imply that the client gets
responsibility for the memory management.
This style is often used when the parent() function returns a
polymorphic object (i.e. an object of a derived class of

Thus in my basic_xml_node class template, i implement the parent()
function to hide pointers (the memory management is implemented by the
library, and client don't need to worry about that).

But that also hides the information that the parent() function returns a
polymorphic object. And your wrapper may even introduce bugs due to
slicing of objects.

template<class XmlNode>
class basic_xml_node {

    typedef basic_xml_node<XmlNode> node_type;

    explicit basic_xml_node(XmlNode* pimpl): pimpl_(pimpl) { }

    node_type parent() {
        XmlNode* px = pimpl_->parent();
        return node_type(px);

For proper copy semantics, you should add a copy constructor like this:

   basic_xml_node(const basic_xml_node& other) :
   { }

You should not just make a shallow copy (a copy of the pimpl_ pointer
itself), but a deep copy (copy/clone the object that the pimpl_ pointer
refers to).

    XmlNode* pimpl_;

Thus, getting the parent node from one node is trivial:

typedef basic_xml_node<LibxmlNode> xml_node; // or, use MsxmlNode
xml_node child = ...;
xml_node parent = child.parent();

I want to keep the API trivial to use, and to disallow client code to
operate directly on pointers. But unfortunately, such implementation
requires that my basic_xml_node is "copyable" (in auto_ptr style).

Any help will be appreciated!


Bart v Ingen Schenau
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